This proposal focuses on an important question in reproduction and chromatin biology: How do metaphase chromosomes condense and become physically distinct ("individualized")? This question will be addressed in the context of gametogenesis and the meiotic mechanisms by which germ cells exit late prophase of meiosis I and enter metaphase of the meiosis I division (the G2/MI transition). Chromatin remodeling processes are crucial for gametes because they set up the alignment of chromosomes on the spindle and ensure their accurate segregation to establish the haploid chromosome content of the future gametes. Remodeling of extended and synapsed prophase chromatin into condensed bivalent chromosomes involves partial condensation and disassembly of the synaptonemal complex and cohesin, followed by further condensation and individualization of chromosomes. The central hypothesis to be tested is that the G2/MI transition requires regulatory phosphatases and activation of kinases, followed by recruitment of condensins and chromatin remodeling to form individualized chromosomes. In Aim 1, control of the localization of nuclear kinases and their inhibitory phosphatases will be determined. Experiments will test the hypotheses that phosphatase inhibition activates aurora kinases, which constitute the G2/MI histone H3 kinase activity and promote the steps of chromosome assembly and individualization. In Aim 2, requirements for the assembly of condensed bivalent chromosomes will be determined to test the hypothesis that ordered assembly of both cohesins and condensins is required for chromosome assembly in both male and female germ cells. In Aim 3, unique mouse mutants with G2/MI arrest phenotypes will be characterized and previously unknown proteins required for the G2/MI transition will be identified. The proposed studies will clarify cell cycle-related mechanisms by which chromatin is remodeled to produce segregation-competent chromosomes, a process that impacts on somatic cell biology and origins of cancer, etiology of aneuploidy and genomic integrity, as well as regulation of fertility and reproductive success. G2/MI "maturation arrest" occurs in many unexplained cases of human male infertility and reproductive toxicity, and thus identifying these regulatory molecules will pinpoint targets in gametogenesis for contraceptive interference and events that, when they go awry, lead to infertility or aneuploidy.